A lithium ion secondary battery is small in volume, has a large mass capacity density, can take out a high voltage, and therefore is widely adopted as a power source for small devices. The lithium ion secondary battery is used as, for example, a power source for mobile devices such as a cellular phone and a notebook-sized personal computer. Moreover, in recent years, application of the lithium ion secondary battery not only to small mobile devices but also to large secondary batteries in the field of electric vehicles (EV), electric power storage, or the like where a large capacity with long life is required has been expected, based on concern for environmental issues and improvement in consciousness of energy conservation.
A positive electrode for lithium ion secondary batteries is constituted from a positive electrode mixture containing: a positive electrode active material such as a lithium composite oxide; a conducting agent such as carbon; and a binder such as a polyvinylidene fluoride (PVDF) and a collector that is joined with the positive electrode mixture.
Examples of the lithium composite oxide that is used for the positive electrode active material include LiCoO2, LiNiO2, LiMnO2, and LiNi1/3Co1/3Mn1/3O2 each having a layered structure and LiMn2O4 having a spinel structure. Among these lithium composite oxides, LiMn2O4 is highly safe and inexpensive and therefore is considered especially suitable as a positive electrode material for large batteries. However, it has sometimes occurred that Mn elutes when LiMn2O4 is exposed to an elevated temperature environment and thereby the deterioration of battery capacity is liable to occur during charge-discharge cycles or storage at elevated temperatures. A method for suppressing the elution of Mn by adding any of various elements to the positive electrode active material to stabilize the crystal structure, or the like has been tried, but it is still hard to say that the problem has fully been solved, and it has remained to be solved when LiMn2O4 is used.
On the other hand, in addition to the improvement in the active material itself, investigation for enhancement of the battery performance such as cycle properties or safety properties has been tried by adding an additive to the electrode or using a binder having a particular structure. The binder plays a role of adhesion between the active materials and between the active material and a collector. For the binder for a positive electrode, the electrochemical stability (oxidation resistance), the resistance to an electrolyte solution, the heat resistance, the slurry properties (imparting viscosity), low cost, and so on are required, and PVDF that is excellent in terms of balance has generally been used.
Moreover, for example, adding a sulfur-containing resin such as polyethersulfone (PES) and polysulfone (PS) to the electrode and using PES or PS as an electrode binder have been known. A method of adding polymer particles of a sulfur-containing resin to the electrode is disclosed in Patent Literature 1, and a method of adding polysulfone, polyethersulfone, or the like to the electrode as an overcharge preventing agent of a lithium nickel manganese positive electrode having a layered structure is disclosed in Patent Literature 2. Moreover, in Patent Literatures 3 and 4, a method of using a sulfur-containing resin for a negative electrode binder is disclosed.